The art is replete with examples for optically reading patterns of uniform size and spacing from planar surfaces of relatively low and unvarying reflectivity, on which the patterns are printed or are otherwise placed in a conventional manner. In general, these patterns are discrete characters belonging to one or more sets of such characters, e.g., to a set of alphabetical characters and a set of numerical characters, each having a predetermined format. Problems arise during the readout of these patterns when, instead of being printed on the pattern-carrying surface, the patterns are raised above the surface by embossing or the like, or when they are impressed into the surface by stamping, engraving, laser cutting, grinding, or by other methods. Such patterns are sometimes referred to as "low-quality" patterns, because they are difficult to read and recognize for reasons associated either with the pattern itself, or with the background from which the pattern must be distinguished in order to be read, or both.
Low quality patterns occur where, for example, the uniformity of pattern size, orientation or spacing cannot be controlled, or where the uniformity of the pattern-carrying surface is not predictable. For example, it may be desired to read line patterns which are serially disposed around the periphery of a cylindrical, specular surface. While optical readout from such a surface is itself difficult, the problem is further compounded if the surface reflectivity varies due to surface blotches or marks, or if surface scratches exist which may be confused with the line patterns themselves.
The problem of reliable readout becomes still more difficult where the patterns do not lie in the surface of the medium, for example where they are stamped or otherwise impressed onto the surface. The stamping of patterns, particularly onto the surface of a metallic medium, may further bring about edge upset, which occurs when the stamping process displaces metal and produces raised ridges along the edges of the stamped line. Where edge upset is present and the uniformity of the surface has been disturbed even by a small amount, the appearance of the pattern image may be altered insofar as an optical pattern recognition system is concerned. Edge upset may cause a pattern to appear enlarged, diminished, or otherwise distorted, so as to produce variations with respect to the predetermined format of the corresponding known pattern to which it must be compared for identification purposes.
Where the stamping or other pattern-applying process is not precisely controlled, the spacing of the respective patterns in a succession, or string, of such patterns may not be uniform. Further, the respective patterns may not be properly aligned with each other along their path on the medium. It is also possible for the orientation of the respective patterns to vary. Finally, any wear of the stamping die, which will be most pronounced in those dies that are more frequently used, may cause the pattern stamped into the surface of the medium to vary in line width, size and shape.
There are many situations in which an optical pattern recognition system must be capable of handling some or all of the above-described variations, yet produce reliable readout of the unknown patterns. Examples of such situations are the reading of type with a view to checking the quality of the type fonts, reading serial numbers from tires in situations where the background of the pattern provides little contrast with respect to the pattern; and reading serial numbers from reflective surfaces, particularly from curved metallic surfaces onto which the line patterns have been stamped.
The last-recited situation poses particularly trying conditions and any or all of the above-mentioned variations may be encountered. For example, serial numbers or other identifying indicia are normally stamped on the cylindrical collars of the end plugs of fuel rods of the type used in a conventional nuclear-fueled power generating station. The serial numbers make it possible to keep track of these rods before and after use. After the number is stamped on the collar of each plug and before the plug is inserted into its fuel rod housing, the plug may or may not be sanded down to reduce the effect of metal upset produced by the stamping process. Plug handling during these operations inevitably produces some scratches on the plug surface which may conceivably be confused with the line patterns during readout. Outgassing during the use of the plug on the rod may create blotches and other marks on the plug surface, which in turn may produce relatively large variations of the reflectivity of the specular plug surface. Finally, even where the plugs are sanded down, some unevenness of the surface due to edge upset is likely to remain and affect the reliability of readout of the patterns.
Heretofore available systems and methods for reading out low-quality patterns of the type described have yielded unacceptably high error rates, ranging as high as 1/100. Some improvement is obtained by carefully monitoring the process by which the pattern is stamped on the end plug surface. However, the relatively small size of the plug diameter, on the order of 1/2 inch, makes accurate control of the stamping process difficult. Further, in a practical situation monitoring cannot be guaranteed, except at relatively high cost. A further disadvantage of applicable prior art optical pattern recognition equipment has been its high cost. This is particularly the case for equipment required to operate under the above-described adverse conditions. As a consequence, there has been a general reluctance to invest in such equipment and a tendency to search for other ways of identifying the fuel rods.
In a copending application of William D. Barber et al, Ser. No. 149,841, filed May 14, 1980, now U.S. Pat. No. 4,339,745 which is assigned to the assignee of the present invention, certain improvements over the prior art in the optical character recognition field are described, which deal with some of the problems outlined above. However, the system and method described in the aforesaid copending application is situation-limited in a number of respects and it requires readout conditions that may not always be present under truly adverse operating conditions.